Night Vision Goggles (NVGs) are devices which enable an aircraft pilot to see images at nighttime illumination levels as low as overcast starlight ambient conditions. All objects radiate and/or reflect energy, however, only a portion of the energy is visible to the unaided eye (i.e., visible spectrum emissions between 360 to 760 nanometers (nm) in wavelength on the electromagnetic spectrum) and at night these energy levels are unviewably low. The ability to sense low level radiant emissions is made possible through the use of NVGs. This capability is achieved by a two-step energy conversion process (light energy to electricity followed by electricity into light) in which the energy received is increased by a factor of 10,000 to 20,000 at maximum sensitivity. This is commonly referred to as the gain or image intensification of the NVG.
The range of NVG spectral sensitivity is limited to 600 nm to 950 nm which encompasses a sector of the visible light spectrum (600 nm to 760 nm) to permit daylight sensitivity as well as a portion of the IR spectrum between 760 nm to 950 nm.
The cockpit lighting that is used in conjunction with NVGs must be such that the pilot can view the instruments while looking under or around NVGs with his unaided eye and must also not degrade the image intensification while looking through the NVGs. Typically, this is achieved by using cockpit light sources that emit light energy with wavelengths from 360 nm to 600 nm (green and blue light) as these wavelengths do not coincide with the NVG spectral sensitivity range (600 nm to 950 nm) and are visible to the naked eye. (See a publication distributed by Jay-El Products, Inc. entitled "Recommendations for Color Limitations of Illuminated Devices Used in Conjunction With AN/AVS-6 Night Vision Goggles" wherein a recommendation is made that only "The colors yellow, green, blue-green, and blue are possible compatible colors for illumination" with NVGs. See also a June 6, 1985 Military Specification (4th draft) entitled "Lighting, Aircraft, Interior, AN/AVS-6 Night Vision Imaging System (ANVIS) Compatible" wherein only Green, Green B and Yellow lighting are specified for compatiblity.)
Typically, cockpits have red warning lights which are believed to be incompatible with NVGs. To eliminate the incompatibility between red warning lights and NVGs, resort has been made to the installation of "flip-up" blue-green filters over the red lights which effectively block the infrared emissions in addition to 40% of the visible emissions. The drawback to this approach is that the "filtered" warning lights no longer draw attention when utilizing NVGs or the unaided eye. Furthermore, the filters must be removed for daytime use. Thus, there exist a variety of problems associated with the use of NVGs in conjunction with light sources.
Accordingly, there is a continual search in this art for lighting that is compatible with NVGs.